1. (Field of the Invention)
The present invention relates to a photosensitive material drum and an image-forming apparatus using the same. More specifically, the invention relates to a photosensitive material drum which permits the photosensitive material to be uniformly worn out over the whole surface thereof, enabling excellent image to be formed and resistance against the printing to be improved.
2. (Description of the Prior Art)
In forming an image by using a photosensitive material drum, a toner image is formed on the surface of the photosensitive material through such processes as electric charging, exposure to image-bearing light and developing, the toner image is transferred, the residual toner is cleaned, and the above-mentioned processes are repeated.
The residual toner is usually cleaned by using a rubber blade leaving, however, a problem in that the surface of the photosensitive material is gradually worn out due to its contact to the photosensitive material drum. It is virtually difficult to uniformalize the force of contact between the surface of the photosensitive material and the cleaning blade over the whole surfaces thereof. Therefore, the surface of the photosensitive material is unevenly worn out depending upon the position of the photosensitive material, and the thickness of the photosensitive layer changes, causing the initially charged potential and the light-attenuated potential to vary, and deteriorating the image and shortening the life of the photosensitive material.
It has heretofore been attempted to uniformalize the wear of the photosensitive material. For example, Japanese Laid-Open Patent Publication No. 63946/1987 discloses a mechanism for uniformalizing the wear of the photosensitive material having means for moving, by a very small amount, the photosensitive material in the direction of the rotary shaft at a timing other than the timing of from a step of exposing the photosensitive material to light to a step of transfer.
The proposal taught in the above-mentioned publication may be effective when the cleaning means and the photosensitive material drum are relatively uniformly contacted to each other in the direction of its length that would be accomplished by using a rubber cleaning blade having a relatively small hardness and when the variation in the wear is limited within a relatively short range in the axial direction of the photosensitive material. However, when the tangential line of contact between the cleaning means and the photosensitive material drum is deviated from the axial direction and, particularly, when the cleaning means is constituted by a rubber blade having a large hardness, it becomes difficult to uniformalize the wear of the photosensitive material drum in the axial direction. Besides, the above-mentioned means requires other constitutions such as a cam and a motor, causing the apparatus to become bulky.
In recent years, it is becoming a common practice to use a toner of a small diameter from the standpoint of improving the resolution of the image. However, the adhering force of the toner to the drum increases with a decrease in the diameter of the toner. Therefore, it is an important technical object to enhance performance for cleaning the toner remaining on the photosensitive material drum.
Performance for cleaning the photosensitive material can be effectively enhanced by increasing the force of contact of the rubber blade to the photosensitive material. In this case, however, the photosensitive layer is scraped in an increased amount accompanying an increase in the force of contact. Besides, an excess of contact force causes the blade to come into contact at its belly portion with the photosensitive material, resulting rather in a decrease in the cleaning performance.
Increasing the hardness of the blade rubber is desirable from the standpoint of preventing the blade from excessively deformed despite the force of contact is increased and converging the peeling force into a line. In this case, however, the pressure exerted on the photosensitive material becomes nonuniform in the direction of length of the photosensitive material, and the photosensitive layer is scraped in varying amounts.
That is, when the cleaning blade is brought into contact with the photosensitive material, the tangential line of contact between the blade and the photosensitive material does not become in perfect parallel with the axis of the drum at a central portion in the direction of length thereof due to allowance of the two members and assembling precision, despite the two members are so brought into contact with each other that a completely contacted state is accomplished. That is, a deflecting angle .theta. necessarily occurs between the two and, hence, the photosensitive layer at the central portion of the drum is scraped off in an amount larger than that at both ends of the drum. Thus, the photosensitive layer is not evenly scraped off.
As the thickness of the photosensitive layer greatly varies, in general, the electrophotographic properties of the photosensitive material undergo a change. For example, there occurs a spot having a low initially charged potential or a spot having a residual potential in the exposed areas, causing the image density to decrease or developing fogging. As a result, the image varies and loses quality. Besides, the photosensitive material loses resistance against the printing and possesses a relatively short life.